The invention relates generally to distributed energy resources.
Electric power is delivered to residential consumers through power distribution systems that typically have radial topologies. In such power distribution systems, feeders branch out from a substation and reach a residential consumer through a step down transformer. Conventional power flow in such axial embodiments is unidirectional from the substation to the residence.
In a typical radial distribution system, voltage sag (drop) along the feeder is proportional to the reactive power on the feeder according to the formula:
            Δ      ⁢                          ⁢      V        =                  PR        +        QX            V        ,wherein ΔV is the voltage drop, P, Q are the real and reactive power, V is the bus voltage; and R, X are feeder resistance and reactance respectively. The resistance R of the feeder line is typically orders of magnitude smaller than the reactance X. Therefore, the voltage drop on the feeder line can be depicted approximately as follows.
      Δ    ⁢                  ⁢    V    ≈            QX      V        .  
For a long feeder line, voltage sag emerges as a significant issue when the reactive load is heavy. One proposed approach to solve the problem is to install capacitor banks at the substation or at pole top capacitors along the feeder line. The theory is that the capacitance load will offset the reactive power consumption on the feeder and that the voltage level at the end of the long feeder can thus be restored. However, this approach often requires extra capital expenditure by the utility. In addition, the capacitor banks are typically switched on and off incrementally. Therefore, continuous coverage of the reactive power compensation can be a challenge.
With emerging distributed energy resources such as residential photovoltaic (PV) systems bi-directionality is becoming more important. As distributed energy resources have increased presence in power distribution systems, advanced functions will be useful to leverage these assets. One such functionality which would be desirable is reactive power (or VAR) support.